Ali Mansour
Brand Manager, 4WD & Sport Utility
Photographers:
James Mathis
Equipping your rig with the correct gear ratios is critical. Most wheelers understand that when you place on much larger-than-stock tires, numerically higher differential gears are needed to compensate for the change. But what do you do when differential gears are not enough? Well, if your 4x4 is equipped with the New Process 231 transfer case, you are in luck.
Found under Jeep Wranglers, Cherokees, Chevy S-10 Blazers and pickups, the NP231 transfer case is one of the most common and easy-to-source transfer cases. From the factory, the NP231 transfer case is fitted with a 2.72:1 low range gearset. In a stock 4x4, this low-range ratio works great, offering enough low-speed control for crawling but not so low you can’t create wheelspin for playing in the sand and mud.
The cast-aluminum housing of the NP231 transfer case and slip-yoke rear output are typically its two major weak points. Fortunately, literally dozens of slip-yoke eliminator kits are available on the aftermarket. As for case protection, depending on the application, typically there are plenty of aftermarket options.
On The Trail
It’s amazing how much such as small sounding gear ratio change can help. The Jeep Wrangler TJ is now more enjoyable to drive off-road, and has the extra torque it desperately needed. We did notice the new gearset was a touch louder than the original set, but the off-road performance tradeoff is worth the audible gear whine.
For those needing more gearing, a great option is the Tera Low 231HD upgrade from TeraFlex. The Tera Low conversion replaces the original front case half of the NP231 case with a cast-aluminum unit fitted with a heavy-duty five-gear 4:1 planetary gearset. Increasing the low-range ratio from 2.72:1 to 4:1 effectively works as a torque multiplier, allowing the Jeep to crawl more easily in low range. To see what it takes to install one of Tera Low units and find out how it works, we grabbed the keys to our buddy’s 2000 Jeep Wrangler.
Equipped with a 2.5L four-cylinder engine, AX-5 five-speed manual, NP231 T-case, locked Dana 44 axles, and 4.88:1 differential gears, the Jeep Wrangler made the perfect candidate for a gearing upgrade. Running a 33-inch-tall tire might not seem like a heavy strain for the small four-popper, but off-road, the Jeep often lacked the bottom end to crawl easily over rocky trails. Giving us a hand with the gearing upgrade is Low Range 4x4 in Wilmington, North Carolina. It took the better part of day from start to finish for the install, but the results were well worth it.
The heart of the transfer case upgrade is the 4:1 gearset. Since the Tera Low 231 HD front case comes with the five-gear planetary gearset already in place, it cuts down on the install time. Our heavy-duty case was built with a 21-spline input to match our AX-5 transmission, but the more common 23-spline units are also available.
For those looking for a two-wheel low-range option, one is available from TeraFlex. Our buddy opted to keep the stock configuration for simplicity.
Since our original NP231 transfer case front output bearing was in good shape, we transferred it to the new housing. We found a hammer and seal driver worked well to install the bearing and new seal.
When you engage the transfer case shifter, a shift fork moves a slide collar and engages the internal gears.
Once the gearsets align, the chain feeds power to the front output, causing the yoke to turn at the same speed as the rear.
The NP231 transfer case chain is the most important part of the transfer case. Over the years, the chain can stretch, causing the transfer case to pop in and out of gear. The chain from our ’00 Jeep Wrangler was in fine working order, so we just gave it a good cleaning and inspection.
After cleaning off all of the old gasket material from the original back half of the case, we applied a bead of gray RTV silicone around the surface mounting area. With the two halves connected, we torqued the case bolts to 30 pound-feet.
One item to be extra cautious of before and after mounting the case halves was the oil pump pickup and feed tube. A small pick helped insure the pump was firmly in place once the rear output cover is attached.
Instead of a mechanical speedo sensor, the newer NP231 transfer case uses an electronic sensor and tone ring. Our case was already fitted with an Advance Adapters slip-yoke eliminator kit that uses the company’s aftermarket shaft and snap rings to keep things in place.
The AA SYE kit allows you to have a stronger fixed rear output. This also gives the Wrangler platform room for a longer aftermarket driveshaft.
You’ll need to double-nut the studs to install them to the new case. To double-nut a stud, thread on one nut, then twist another nut atop it. Using a wrench, tighten the two nuts against each other, so the top nut now acts like a bolt head.
The new TeraFlex 231 HD setup is the same length and width as the stock NP231. This means you can retain your drivelines and stock transfer case shift linkage.
